Superheated water
Solubility of organic materials and gases increases by several orders of magnitude and the water itself can act as a solvent, reagent, and catalyst in industrial and analytical applications, including extraction, chemical reactions and cleaning.
A significant effect on the behaviour of water at high temperatures is decreased dielectric constant (relative permittivity).
Despite the reduction in relative permittivity, many salts remain soluble in superheated water until the critical point is approached.
For nitrogen, this minimum is 74 °C and for oxygen it is 94 °C[5] Gases are soluble in superheated water at elevated pressures.
Corrosion of an Inconel U-tube in a heat exchanger was blamed for an accident at a nuclear power station.
[8] Therefore, for occasional or experimental use, ordinary grades of stainless steel are probably adequate with continuous monitoring, but for critical applications and difficult to service parts, extra care needs to be taken in materials selection.
[2] Extraction using superheated water tends to be fast because diffusion rates increase with temperature.
Therefore, extraction with superheated water can be both selective and rapid, and has been used to fractionate diesel and woodsmoke particulates.
[13] Superheated water is being used commercially to extract starch material from marsh mallow root for skincare applications[14] and to remove low levels of metals from a high-temperature resistant polymer.
Organic compounds are rapidly oxidised without the production of toxic materials sometimes produced by combustion.
An example of a selective reaction is oxidation of ethylbenzene to acetophenone, with no evidence of formation of phenylethanoic acid, or of pyrolysis products.
Thermal depolymerization or thermal conversion (TCC) uses superheated water at about 250 °C to convert turkey waste into a light fuel oil and is said to process 200 tons of low grade waste into fuel oil a day.
A demonstration plant is due to start up in The Netherlands said to be capable of processing 64 tons of biomass (dry basis) per day into oil.
Since the polarity of water spans the same range from 25 to 205 °C, a temperature gradient can be used to effect similar separations, for example of phenols.
[26] The use of water allows the use of the flame ionisation detector (FID), which gives mass sensitive output for nearly all organic compounds.
